When food scientist say an apple has 100 calories, they are specifying the potential energy (heat) that the apple can generate as it passes through the body. Each food calorie is equal to the amount of energy required to raise the temperature of 1 kilogram (1 liter or 2.2 pounds) of water by 1C at 1 atmospheric pressure. This calculation is based on a kilogram rather than a gram, so a food calorie is 1,000 times greater than the normal calorie, the one commonly used in physics and chemistry.

Scientists calculate the food-calorie value with devices such as the bomb calorimeter, a sealed compartment in which a specific weight of food is completed burned. Sensitive instruments measure the amount of energy generated by the burning food.